The lac repressor and the RNA polymerase can be cross-linked to DNA that contains bromouracil in place of thymine. Ultraviolet light irradiation knocks off the bromine and creates a free radical which reacts with the protein. We hope to identify which amino acids in the lac repressor are close to which bases in the operator, in order to begin to form a three-dimensional picture of that specific interaction. We wish to identify which subunits of the RNA polymerase interact with DNA and to gain some picture of the DNA regions that those subunits contact.